The present invention relates generally to a Pulse-Width Modulator (PWM) control circuit for controlling the electrical current level of a modulated output signal of a PWM circuit. More particularly, the invention relates to a circuit for controlling the current supplied to a load device, or devices, during a start-up period of the device(s) as well as subsequent to the start-up period.
A PWM is a device that can provide the necessary drive signals required by various types of electronic devices. A PWM circuit works by generating a square wave with a variable ON/OFF ratio, the average ON time may be varied from 0 to 100 percent. In this manner, a variable amount of power is transferred to the device being driven.
A PWM voltage conversion system offers a relatively simple control algorithm, simple circuit structure and low cost. A PWM controller is typically configured to drive one or more main switches, such as a Field-Effect Transistors (FET), and provides a pulsed voltage signal to the transistor. When the pulsed voltage signal is in one state, i.e., a high state, the transistor is turned ON, allowing current to flow to the load. Alternatively, when the pulsed voltage signal is in another state, i.e., a low state, the transistor is turned OFF and current is prevented from flowing to the load. The ratio of ON time to signal period is referred to as the duty cycle of the drive signal. The duty cycle of the PWM drive signal, thus, determines the average amount of current and, hence, power supplied to the load.
For the most part, when designing a PWM drive circuit, the main consideration lies in the simplicity of the design. This is not necessarily the case, however, when the PWM converter is responsible for supplying power to, for example, tungsten filament lamps or DC motors.
In regard to tungsten filament lamps, due to the physical characteristics of tungsten, the resistance of the filament when it is cold is much smaller than its resistance when the tungsten is hot. Accordingly, during a start-up period, when the lamp is first energized and the filament is cold, the small cold resistance leads to a large inrush current in the filament. For example, in a situation where a 42/14 volt PWM converter is used, the inrush current to the filament could be 70 amps, due to the amplitude of the voltage pulses, i.e., 42 volts, as opposed to an inrush current of 25 amps for systems using a 14 volt PWM converter. The huge inrush current could significantly shorten the useful life of the lamp and may lead to big voltage fluctuations on the power bus.
Accordingly, in order to add usable life to filament lamps, there have been attempts to limit the inrush current to the filaments. One method provides a current control means to the conventional 42/14 volt PWM converter. According to this method, the inrush current can be limited to a level that is about the same as that in a 14 volt DC system, thereby assisting in prolonging the lifetime of the lamp.
A PWM is used to provide pulsed voltage or current to the switch driver. Compared to a DC system, during the start up period, the pulsed energy supplied by a PWM system in a unit time is smaller. The smaller power, however, introduces longer start up time, i.e., the time from the start of lamp energization until the lamp is fully illuminated. In some applications, i.e., automotive break lamps, a long start up time leads to a delay in the lamp providing whatever it is that it is intended to indicate. This condition is not acceptable for many applications and represents another significant drawback in conventional PWM conversion systems.
Similar drawbacks to the ones mentioned above for tungsten filament lamps occur when PWM converters are used to drive DC motors. For example, in a 42/14 volt system, where a 42 volt DC power supply and a 14 volt DC motor are used, the large voltage supplied from the power supply causes a large inrush current to flow in the motor. This large inrush current can cause a shock to the mechanical structures and/or large interference in the electrical systems associated with the motor. By limiting the output of the PWM converter in a similar fashion to the method for limiting the output for the lamp, discussed above, the start up of the motor is delayed, which can cause disastrous effects.
Therefore, a system whereby a low inrush current during start up is supplied and then control is returned to the PWM controller is desired.
In view of the aforementioned problems with the conventional approach to PWM converter control, the present invention comprises a simple control circuit that provides a short start up time for devices such as tungsten filament lamps and DC motors, provides a low, controlled, inrush current to these devices and, automatically detects the end of the start up period and returns control back to the PWM controller.
In order to achieve the main objective of the invention, a preferred embodiment in accordance with the present invention provides a device comprising a controller operable to receive input voltage signals and generate a PWM output signal in accordance with a predetermined duty-cycle, a current control device operable to receive the PWM output signal and generate a driver input signal. The current control device can further comprise a start-up detector operable to determine when a start-up time for the load has elapsed and a start-up time limit device operable to set up a start-up animate time and maintain the start-up detector in an operable state until the start-up animate time has elapsed. The device may also include a driver that receives the driver input signal and outputs a driver output signal that is a modified version of its input signal, and a switch device connected to the driver that can receive the driver output signal and supply power to the load in accordance with the driver output signal.
Prior to a start-up time being detected, a control signal from the controller is masked by a chopping signal that is synchronous with the current being drawn by the load as the current fluctuates between a minimum level and a maximum level. Thus, during the start-up time, the current drawn by the load is not permitted to exceed a certain amount. However, because the load is supplied with pulses of energy, wherein the level of the pulses are not reduced, as in the conventional art, the start-up time of the load is not significantly reduced. After the start-up time of the load has been reached, the controller is once again handed control and the standard PWM control signal generated by the controller is used to supply the load with current. Accordingly, a fast start-up time can be achieved without the detrimental effects of a large current being drawn through the load during start-up.